An inkjet ink composition generally consists of a vehicle, which functions as a carrier, and a colorant such as a dye or pigment. Additives and/or cosolvents can also be incorporated in order to adjust the inkjet ink to attain the desired overall performance properties.
In general, pigments alone are not readily dispersible in liquid vehicles. A variety of techniques have been developed which can provide stable pigment dispersions which can be used in inkjet printing. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants include water-soluble polymers and surfactants. Typically, these polymeric dispersants have a molecular weight less than 20,000 g/mol in order to maintain solubility and therefore pigment stability.
Modified pigments have also been developed which provide ink compositions with improved properties, such as dispersibility, without the need for an external dispersant. For example, U.S. Pat. No. 5,851,280 discloses methods for the attachment of organic groups onto pigments including, for example, attachment via a diazonium reaction wherein the organic group is part of the diazonium salt. The resulting surface-modified pigments can be used in a variety of applications, such as inks, inkjet inks, coatings, toners, plastics, rubbers, and the like. These modified pigments provide inkjet inks with good overall properties.
Polymers other than dispersants have also been incorporated into inkjet ink compositions in an effort to improve performance. For example, emulsion polymers or latexes, which are typically high in molecular weight, have been used as an additive to improve the print performance of inkjet inks. However, as emulsion polymers, these polymers are not soluble and would tend to lead to difficulties in printing, particularly nozzle clogging.